drm/amd/display: SPL cleanup

[Why & How]
Move from pointer to callback to reference callback directly
Missed renaming fixpt functions with spl prefix

Reviewed-by: Navid Assadian <Navid.Assadian@amd.com>
Signed-off-by: Samson Tam <Samson.Tam@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>